Support structures for overhead directional signs are commonly placed in overlying relation to vehicular travel lanes along major highways. These structures are subject to the wide and varying design and actual loading conditions typically encountered, such as wind loads, ice loads, live loads, and varying combinations of such. The failure of such an overhead structure, in which case it would likely fall into the pathway of speeding traffic, could be disastrous with consequent loss of life and injury to vehicular passengers. Thus, the dependability of overhead sign support structures used in this environment is critical.
Vehicular sign support structures are exposed to the havoc of outdoor corrosion that can be aggravated by the corrosive atmosphere prevailing in coastal and metropolitan regions. The weight of the driver information sign or sign supported is not necessarily insignificant. Moreover, such signs typically present large sail areas exposed to heavy and irregular wind loads eminently capable of inducing powerful vibrations in the support structure. Typically, such horizontal support structure is vulnerable to vibration as it extends long distances across multiple traffic lanes with corresponding vulnerability to rapidly changing dynamic loads induced by wind and vibration.
One of the causes of failures for these structures may be  traced to metal fatigue. The nature of fatigue crack induced via weldment attachment imperfections is well known. In metallic truss structures for use as girders, frames or other purposes, parts are generally secured together by electrically welding them at the points of intersection. Due to the softening of the metal and the application of pressure in the welding operation, the edges of the metal components being welded together bite or sink into each other slightly at these points of welded contact. The resulting metallurgic and physical discontinuities resulting from the welding process has been identified as one of the significant causes of fatigue load susceptibility.
Due to the fact that a welded, riveted or other type joint, under tension, is dependent wholly upon the amount of material used at the joint (either welded or riveted), such joint as compared to a similar joint for compression members is weak, inefficient, and incapable of long withstanding distresses carried thereto.